Abstract In solid organ transplantation, immunosuppressive therapy has significantly improved short-term organ allograft survival by reducing acute rejection rates. However, chronic rejection - mediated by T cells, antibodies (Abs), or both - has not declined in incidence and remains an important obstacle to long-term allograft survival. A likely, important contributor to the pathogenesis of chronic rejection is the formation of tertiary lymphoid organs (TLO) within the graft. Evidence that TLO play a causative role in rejection derives from both mice and humans, as TLO have been documented in a majority of chronically rejected mouse and human allografts, and experimental models have shown that they support nave T and B cell activation and influence graft outcome. Moreover, analysis of human renal allograft biopsies has demonstrated B cell hypermutation and Ab production within TLO. Together, these studies support the hypothesis that TLO are niduses of local (intragraft) immune activation in chronic allograft rejection. In Aim 1 of this grant application, we will establish the cause-effect relationship between TLO and chronic renal transplant rejection in the mouse. In Aim 2, we will investigate the cellular and cytokine mechanisms responsible for TLO formation in allografts, with particular emphasis on innate lymphoid cell (ILC) subsets. In Aim 3, we will delineate the specific immunologic functions of TLO in chronic rejection, with focus on B cell activation and antibody production. The proposed studies address the unmet challenge of treating and preventing chronic rejection and will also shed light on other conditions in which TLO play a prominent role, such as autoimmunity and cancer.